Shortening Boat Trailer

**nvn** · Dec14-09, 01:00 AM

pjm: I envisioned something like the following, using eight aluminum 6061-T6 plates. See attached file.

D = bolt nominal diameter = 9.525 mm
a1 = a3 = 31 mm
a2 = a4 = 35 mm
s1 = 42 mm
s2 = 57 mm
a5 = 22 mm
s3 = 44.5 mm
a6 = 25 mm
s4 = 60 mm
Plate thickness, t1 = t2 = 6 mm, t3 = 5 mm.

0.3750-16 UNC, SAE grade 8 bolts, grade 8 nuts, and grade 8 flat washers; installation torque, T = 50 N*m (37 ft*lbf) above running torque of locking device, if unlubricated. If lubricated, let me know.

**pjm** · Dec14-09, 01:23 AM

NVN: I thought I had replyed back this morning, but something must have happened. This is what I was looking for and I thank you very much. The only thing I might think about changing would be to replace the top plates with one triangular shaped plate covering both i-beams and the area between (about 2'x3' made out of 1/4" aluminum checker plate and bolted through both sides of flanges on both beams as you show. Thanks again for your time and patience. PJM

**dacarls** · Dec14-09, 08:27 AM

Now you are getting somewhere. Trimming edge of girders to 4.91 inches from 5 inches, while adding 1/4 inch Al plate - A problem? not to me, the "amateur but practical engineer". Just put another small piece of Al plate for a spacer on top of the (removable) tongue when it is replaced and before the tower bolts are retightened. This keeps it from collapsing 1/4 inch down onto the tongue.

**nvn** · Dec14-09, 10:39 AM

pjm: Checker plate is too weak. It cannot be used as a replacement for the I-beam splice top plate. I-beam splice components are primary structural components, whereas checker plate is not. Furthermore, the rough surface of checker plate would cause uneven seating of the I-beam splice washers; the washers need to be perfectly level on a flat, smooth surface, to prevent prying on the bolt head.

**pjm** · Dec14-09, 12:24 PM

Originally Posted by nvn

pjm: Checker plate is too weak. It cannot be used as a replacement for the I-beam splice top plate. I-beam splice components are primary structural components, whereas checker plate is not. Furthermore, the rough surface of checker plate would cause uneven seating of the I-beam splice washers; the washers need to be perfectly level on a flat, smooth surface, to prevent prying on the bolt head.

NVN: OK I get the picture. Incidently, what was the reason for reducing the bolt size in your last message. Thanks again
PJM

**nvn** · Dec16-09, 11:15 AM

pjm: The smaller the bolt size, the better (if adequate), because you want to remove as little of the I-beam flange material as possible. Even 0.3125-18 UNC, grade 8 bolts, torqued to 28 N*m (unlubricated), might work, and might be better, because they would remove less flange material. Or, if you use 0.3750-16 UNC bolts, intuition tells me you could probably omit one column of the bolts shown in post 49, which would still be twelve bolts holding each I-beam, instead of eighteen. And t1 = t2 = t3 = 5 mm might work.